GUESTFSD_EXT_CMD was used by OpenSUSE to track which external commands
are run by the daemon and package those commands into the appliance.
It is no longer used by recent SUSE builds, so remove it.
Thanks: Pino Toscano, Olaf Hering.
Introduce a new API to create a new squashfs filesystem out of a path
in the guest. It can be configured to exclude paths based on patterns,
and to select which compression use for the filesystem.
The advantage of running mksquashfs directly in the appliance is that
ownerships are properly saved, as opposed to tar_out + local untar.
